Ceramic heater roller with thermal regulating layer

Abstract

A thermal conduction roller for use in industrial and office equipment includes a ceramic heater layer and a second ceramic layer disposed in between the ceramic heater layer and the core. The second ceramic layer has a resistance characteristic such that resistance of the roller decreases with heating to approximately the same order of magnitude as resistance in the longitudinal direction. This causes electrical current to be diverted from the ceramic heater layer to limit the temperature in that layer and any outer layer contacting a work object. The resistance of the second ceramic layer is controlled in part by controlling the parameters of plasma spraying process, including hydrogen secondary gas flow.

I claim: 
     
       1. A thermal conduction roller for use in a machine in which a voltage is applied to the roller to cause heating within a heating layer, the roller comprising: a cylindrical roller core having a cylindrical surface;   a ceramic heating layer having a resistance for resistive heating, the ceramic heating layer being disposed over the cylindrical surface of the roller core;   means for connection of the ceramic heating layer to an electrical source to enable current to flow through the ceramic heating layer to produce heat, some of the heat being conducted outwardly to an outer surface of the roller; and   a ceramic heat regulating layer disposed between the core and the ceramic heating layer, the heat regulating layer being less conductive than the heating layer at lower temperatures, and the resistance of the heat regulating layer decreasing in a radial direction relative to the roller as temperature increases over a respective temperature range, such that current is diverted from the ceramic heating layer to regulate temperature in the ceramic heating layer.   
     
     
       2. The roller of claim 1, further comprising an outermost layer, which forms the outer surface of the roller, the outermost layer being disposed for contacting and conveying heat to a work object.   
     
     
       3. The roller of claim 1, wherein the cylindrical roller core includes an outer surface of conductive material; and   wherein the conductive material is connected to electrical ground so that current is diverted from the ceramic heating layer through the heat regulating layer and core.   
     
     
       4. The roller of claim 3, wherein the roller core is formed completely of conductive material. 
     
     
       5. The roller of claim 1, further comprising a plurality of conductive bands formed on the ceramic heating layer and spaced apart to provide bypasses for current flowing longitudinally through the ceramic heating layer. 
     
     
       6. A thermal conduction roller for use in a machine in which a voltage is applied to the roller to cause heating within a heating layer, the roller comprising: a cylindrical roller core having a cylindrical surface;   a ceramic heating layer disposed over the cylindrical surface of the cylindrical roller core;   means for connection of the ceramic heating layer to an electrical source to enable current to flow through the ceramic heating layer to produce heat, some of the heat being conducted outwardly to an outer surface of the roller;   a ceramic heat regulating layer disposed between the core and the ceramic heating layer; and   wherein the ceramic heat regulating layer is a plasma sprayed blend of first and second ceramic materials having a resulting electrical resistance which decreases in a radial direction relative to the roller as temperature increases to divert electrical current and tend to limit temperature in the ceramic heating layer.   
     
     
       7. The roller of claim 6, wherein the electrical resistance of the ceramic heat regulating layer is controlled by blending a first ceramic material with a second ceramic material and applying the ceramic heat regulating layer in a thickness selected to control electrical resistance.   
     
     
       8. The roller of claim 7, wherein the first ceramic material is titanium dioxide or chromium oxide; and   wherein second ceramic material is alumina or zirconia.   
     
     
       9. The roller of claim 6, wherein the ceramic heat regulating layer is made by plasma spraying a blend of titanium dioxide and another material; and   wherein the electrical resistance of the ceramic heat regulating layer is controlled by varying the hydrogen secondary plasma gas level.   
     
     
       10. The roller of claim 6, further comprising a plurality of conductive bands formed on the ceramic heating layer and spaced apart to provide bypasses for current flowing longitudinally through the ceramic heating layer.